Intermetallic compounds based on the general formula AB.sub.5 form an important class of hydrogen storage materials. In general, A is a rare earth element and B is usually Ni or Co, often with other minor transition metal substitutions. The classic AB.sub.5 hydrogen storage compound is LaNi.sub.5.
The rare earth elements never appear singly in nature but rather as mixtures. Examples are bastnasite and monazite ores which contain mixtures of primarily Ce, La, Nd, Pr and a multiple of other rare earths and chemically similar elements (e.g., Y and Th). Because of the expense (both in processing and inventory) of isolating La from the natural rare earth mixtures, LaNi.sub.5 must always be relatively expensive. The direct use of rare earth mixtures (R) to form RNi.sub.5 hydrogen storage compounds is required for cost reductions.
One approach has been to use the naturally occurring mixture of rare earths, mischmetal. For example, bastnasite mischmetal consists of 48-50%Ce, 32-34%La, 13-14%Nd, 4-5%Pr and about 1.5% other rare earths. In fact, hydrogen storage alloys based on bastnasite mischmetalnickel (e.g. the HY-STOR.TM. alloys) are commercially available today. Usually a further substitution for R (e.g., Ca) or B (e.g., Al, Fe, Mn, etc.) is made in order to reduce the high plateau pressure and hysteresis of RNi.sub.5 hydrides containing natural Ce-rich mischmetals.
At first glance, it might appear that natural mischmetal would be the ultimate R component for RNi.sub.5 production. This is not necessarily so. First, the composition of mischmetal varies with time and geographic source, which in turn can lead to variations in RB.sub.5 hydride properties. For example, monazite mischmetal has different ratios of the rare earth elements than bastnasite mischmetal. Second, short term demands for individual rare earths can result in their removal in processing with the final by-product having a mixture of rare earths quite different from the original one.
Alloy manufacturers must have the flexibility to use a spectrum of rare-earth mixtures for the production of RNi.sub.5 hydrogen storage compounds if they expect to efficiently and economically utilize the lower cost products offered by the rare-earth industry. It is the object of this invention to provide a means whereby this objective can be readily attained.